Abrasion resistant austenitic stainless steel and process for making same



April 8, 1969 H. E. MCCUNE m ABRASION RESISTANT AUSTENITIC STAINLESS STEEL AND PROCESS FOR MAKING SAME Sheet Filed May 5. 1965 INVENTOR Harry E. McCune Ill 44 W3 ATTORNEY 3,437,477 TANT AUSTENITIC STAINLESS STEEL Sheet 2 of 7 April 3, 1969 H. E. M CUNE m ABRASION RESIS AND PROCESS FDR MAKING SAME Filed May 5. 1965 FIG INVENTOR Harry E. McCune Ill BYw ATTORNEY April 8, 1969 H. E. MQCU-NE m 3, 3

ABRASION RESISTANT AUSTENITIC s'mnmss STEEL AND PROCESS FOR MAKING SAME Filed May 5. 1965 Sheet 3 of 7 INVENTOR Harry E. McCune lll Man 7/14 ATTO R NEY p 8, 1969 H. E. M CUNE m 3,437,477

ABRASION RESISTANT AUSTENITIC STAINLESS STEEL AND PROCESS FOR MAKING SAME Filed May 5. 1965 Sheet 4 of 7 INVENTOR Hurry E. McCune Ill BY mm W ATTORNEY April 8, 1969 H; E. M CUNE 111 3, 37,477

ABBASION RESISTANT AUSTENITIC STAINLESS STEEL AND PROCESS FOR MAKING SAME Filed May 5, 1965 Sheet 5 of '7 FIG. 5

INVENTOR Harry E. McCune Ill MMM w y ATTORNEY pril 8, 1969 H. E. M CUNE m 3,437,477

, ABRASION RESISTANT AUSTENITIC STAINLESS STEEL AND PROCESS FOR MAKING SAME Filed May 5, 1965 Sheet 6 of"? INVENTOR Harry E. McCune lll ma, WW

ATTORNEY 3,43 7,477 namsxon RESISTANT AUSTENITIC smmmss smm,

Sheet 7 of 7 April 8, 1969 H; E. M CUNE m AND PROCESS FOR MAKING SAME Filed May 5. 1965 FIG. 7

INVENTOR Harry E. McCune lll Zz/MWW ATTORNEY United States Patent M 3,437,477 ABRASION RESISTANT AUSTENHTKC STAINLESS STEEL AND PROCESS FOR MAKING SAME Harry E. McCune III, Huntington Woods, Mich., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Filed May 5, 1965, Ser. No. 453,897 Int. Cl. C22c 41/00, 39/22; C2111 7/02 US. Cl. 75-128 Claims ABSTRACT OF THE DISCLOSURE The present invention describes an abrasion resistant, austenitic stainless steel in which the carbon content exceeds the solubility limit of carbon in the austenitic matrix at 2100 F. Various processing and heat treatments are described to produce massive carbides randomly distributed, smaller discontinuous carbides precipitated at the grain boundaries, and fine carbides distributed principally on the slip planes.

This invention relates to abrasion resistant steels, and more particularly to an improved abrasion resistant austenitic stainless steel and a method of manufacturing such a steel.

There is a constant quest going on for steels which have a very high degree of abrasion resistance combined with a good degree of formability in order that finished parts may be readily manufactured which have the required abrasion resistance. In addition, many applications require that the steel have a very high resistance to corrosion, inasmuch as the parts formed therefrom are often required to be used in corrosive atmospheres. In addition to the desired degree of abrasion resistance, corrosion resistance and formability, it is necessary that the steels have suitable hot and cold working characteristics so that they may be economically produced. These types of steels can find ready application in wire screens through which will pass abrasive particles, and certain types of cutlery and food-cutting equipment, as well as a variety of other uses.

It is therefore a principal object of this invention to provide an improved abrasion resistant steel.

Yet another object of this invention is the provision of an improved abrasion resistant austenitic stainless steel which has suitable cold forming characteristics.

Still a further, more particular object of this invention is the provision of an improved austenitic stainless steel having a microstructure in which carbides are distributed in an austentic matrix in a manner to provide improved abrasion resistance with adequate formability, and maintain good corrosion resistance of the steel.

Yet another, more general object of this invention is the provision of a modified austenitic stainless steel and a method of processing and treating said steel to provide improved abrasion resistance and adequate formability while maintaining good corrosion resistance.

Yet another, more specific object of this invention is the provision of a modified austenitic stainless steel and a method of processing such steel to provide a distribution of carbides in an austenitic matrix which will result in improved abrasion resistance of the steel combined with good cold formability and adequate corrosion resistance, and which process can be economically practiced.

These and other objects, together with a fuller understanding of the invention, will become apparent to one skilled in the art when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is an electron micrograph, at 25,000 of 3,437,477 Patented Apr. 8, 1969 a plastic replica of the microstructure of an austenitic stainless steel according to this invention, after cold rolling and prior to the carbide precipitation treatment, the sample having been etched in a mixture of acids containing 10 parts nitric, 10 parts acetic, 15 parts hydro chloric and 1 part glycerine, known as mixed acids;

FIG. 2 is an electron micrograph, at 25,000X, of a plastic replica of the microstructure of an austenitic stainless steel according to this invention, after the carbide precipitation treatment for five minutes at 1300 F. following cold rolling, the sample having been etched in the mixed acids used for etching the sample of FIG. 1;

FIG. 3 is an electron micrograph, at 25,000X, of a plastic replica of the microstructure of the austenitic stainless steel of this invention after cold rolling and prior to carbide precipitation treatment, the sample having been etched in a mixture of 5 ml. hydrochloric acid, 1 g. picric acid and ml. methyl alcohol;

FIG. 4 is an electron micrograph, at 25,000 of a plastic replica of the microstructure of the austenitic stainless steel of this invention after the carbide precipitation treatment for five minutes at 1300 F. following cold rolling, the sample having been etched the same as the sample of FIG. 3;

FIG. 5 is an electron micrograph, at 10,000 of a plastic replica of FIG. 4;

FIG. 6 is an electron micrograph, at 10,000X, of a plastic replica of the microstructure of the austenitic stainless steel of this invention after the carbide precipitating treatment for two minutes at 1300 F. following cold rolling, the sample having been etched the same as the sample of FIG. 3, and

FIG. 7 is an electron micrograph, at 25,000X, of a plastic replica of the microstructure of the austenitic stainless steel of this invention after a carbide precipitating treatment of 30 seconds at 1300 F. following cold rolling, the sample having been etched the same as .the sample of FIG. 3.

I have found that by modifying the composition of austenitic stainless steels and by providing a proper cold Working and heat treatment to such modified steel, an austenitic stainless steel having improved abrasion resistance, and also having good formability and corrosion resistance, can be produced. The term austenitic stainless steel, as used herein, is intended to refer to that class or group of steels which, in the fully annealed condition, have a predominantly or Wholly austenitic microstructure. These steels contain up to about 16% manganese, up to about 2% silicon, from about 10% to about 25% chromium, up to about 20% nickel, up to about .40% nitrogen, with the balance iron and residual impurities. Certain other elements such as molybdenum, columbium, titanium, tungsten, vanadium, boron, copper, zirconium, selenium and sulfur, as well as other elements, may be added to obtain certain properties in austenitic stainless steels. It is well known, however, that upon cold working, certain types of austenitic stainless steels will produce some martensite. This characteristic of martensite formation is dependent upon the balance in the steel of austenite formers and ferrite formers, and upon the degree of cold work to which the steel is subjected. For example, AISI Type 301 stainlesssteel, as well as AISI Type 201 stainless steel, classified and considered austenitic stainless steels, will form substantial amounts of martensite even with moderate cold Working, whereas Types 302 and 304 require a substantial amount of cold working to form martensite and Type 305 will form no appreciable martensite even upon heavy cold working. It is therefore to be understood that the term austenitic stainless steel, as used herein, is intended to apply, as noted above, to those grades which are Wholly or predominantly austenitic in the fully annealed condition.

The invention as described herein will be described in conjunction with a modified AISI Grade 304, although it is to be understood that it is equally applicable to other grades of stainless steel as defined hereinabove.

I have found that by increasing the carbon content in A151 Type 304 to above the amount which is soluble in the austenitic matrix of the steel at 2100 F., and preferably to between .40% and 50% and properly processing this steel, an austenitic stainless steel having improved abrasion resistance with good formability and corrosion resistance will be provided. The stainless steel is melted in a conventional manner with the alloying elements, other than carbon, controlled to Within the limits which will provide an austenitic matrix in the fully annealed condition. The carbon content is controlled to preferably between .40% and 50%. The steel is hot rolled in a conventional manner to any desired intermediate form and gauge, depending upon the end product desired. Following the hot rolling, the material is alternately cold worked and annealed above the recrystallization temperature until the final gauge is reached, after which the carbide precipitating treatment is performed. The following is a sample of one process that can be followed to provide cold drawn and roll flattened wire. A stainless steel having substantially the composition of A151 Type 304 with between .45% and .50% carbon is melted and hot worked to A rounds. These rounds are annealed between 1900 and 2100 F. and water quenched. They are then pickled and coated with drawing compound and drawn to .150 rounds. They are then batch annealed between 1800 and 1900 F., water quenched, pickled, coated with drawing compound and drawn to .100" rounds. These rounds are again batch annealed at 1800 to 1900 F., water quenched, pickled, coated with drawing compound and drawn to .090" rounds. Following this cold drawing operation, the wire is strand annealed at approximately 1900 F. and rolled to a flat wire .024" x .153". The electron micrograph of FIG. 1 shows the microstructure of a sample of this material following this roll flattening. As can be seen, there are massive randomly distributed carbides throughout a predominantly austenitic matrix. It should be noted that after the roll flattening operation, a substantial amount of cold working has been performed on the steel and some martensite is formed. Magna gauge testing has indicated that there may be of the order of magnitude of about 6% martensite present. Still referring to FIG. 1, various sets of parallel striations can be observed, and these striations are slip planes within the austenite crystals of the matrix of the steel. These slip planes have been formed by the roll flattening operation. As can be seen from FIG. 1, there is no evidence of any precipitate in these slip planes.

Following the roll flattening operation, the wire is subjected to a carbide precipitating treatment in the carbide precipitating range of from about 800 F. to about 1700 F. However, if good strength is to be maintained, this precipitation treatment should be carried out below the recrystallization temperature of the austenite. This recrystallization temperature depends upon the exact composition of the steel, the amount of cold work to which the steel has been subjected, and the time at which it is held at temperature. For the type of steel described in this example and with the amount of cold working as described, if the steel is held for five minutes or less at 1300 F. there will be no recrystallization produced. It is desired, however, to utilize a temperature as high as possible for this carbide precipitating treatment to increase the rate of precipitation of the carbides, inasmuch as the rate of precipitation is a time-tcmpreattire-dependent variable.

The steel after roll flattening was given a carbide precipitation treatment at 1300 F. for five minutes. This caused a large amount of carbide precipitation, predominantly at the slip planes produced by the cold working operation. This carbide precipitation at the slip planes can be seen in FIG. 2. Referring now to FIG. 2, the large, massive carbides 10 are still evidenced in a randomly distributed manner, but also evident are very small carbides which are distributed in parallel fashion. These carbides are precipitated on the slip planes which were formed and are evident in FIG. 1. These slip planes provide excellent nucleation sites for carbides to precipitate, and this fine carbide precipitate is present throughout the crystals and grains of the matrix. There is also evidenced some grain boundary carbide precipitation, which carbides are smaller than the massive carbides but larger than the very small carbides which are precipitated on the slip planes. It should be noted, however, that the grain boundary carbides are not continuous, i.e. they do not form a continuous carbide network, but rather are discontinuous. It is desired to avoid continuous carbide networks in the grain boundaries since these carbide networks are susceptible to grain boundary corrosion, known as intergranular corrosion. It thus can be seen that when a supersaturated austenitic stainless steel (i.e. a stianless steel having more carbon than is soluble in austenite) is processed according to this invention wherein a cold working treatment is given the material to provide slip planes and following this cold working treatment the material is heated in the carbide precipitating range, a microstructure is provided which contains a predominantly austenitic matrix with randomly distributed massive carbides, with discontinuous smaller carbides at the grain boundaries and with fine carbides located principally on the slip planes caused by the cold working. This structure of cold worked austenite with this distribution of carbides provides a material which can be rather easily cold formed to desired shapes, and yet wihch will be extremely abrasion resistant. Although it is preferred that the carbide precipitating treatment he performed after the final cold rolling step, this treatment may be performed after any of the cold working steps in the processing, provided suflicient reduction has been given the material to provide slip planes upon which to precipitate the carbides. Although slip planes may be produced with as little as 3 or 4% reduction, it is preferred that at least 10% reduction in area of the material be made in order to provide a sufliicent number of nucleation sites to yield the desired distribution of carbides. It may, in some instances, therefore, be desired to perform the carbide precipitating treatment the step prior to the final cold working step if the material has remaining sufficient ductility that it can be worked to final guage. For example, prior to the roll flattening and after the cold draw to .090" in the example listed above, it may be desirable to give the material a carbide precipitating treatment in the range of 1000 F. to 1300 F. If, on the other hand, at a step prior to the final cold working step the material has been cold worked in an amount so that further cold working without cracking would be impractical, the carbide precipitating treatment can be accomplished above the recrystallization temperature, in which case as the material passes through the carbide precipitating range, carbides will be precipitated on the slip planes, after Which the material will recrystallize with the finely dispersed carbides remaining distributed through the matrix. The material in this condition can then be cold worked to final gauge, which cold working will add to the strength of the austenite matrix and the fine carbides will be redistributed, but nevertheless dispersed, throughout the matrix.

Referring now to FIGS. 3, 4 and 5, the same material as shown in FIGS. 1 and 2 is depicted using a somewhat different type of etchant. The etchant used for these FIGS. 3, 4 and 5 is a mixture of hydrochloric acid and picric acid and will attack and remove fine and medium sized carbides. This acid, for the times used, will attack around the massive carbides but will not remove them.

The electron micrograph of FIG. 3 shows the steel after it has been roll flattened and before the carbide precipitation treatment. Here again are evidenced the massive carbides 10, but there are no small carbides in evidence.

FIG. 4 shows the material after a five-minute carbide precipitating treatment at 1300 -F. The massive carbides 10 are still in evidence, but also a great number of parallel grooves are shown from which the small carbides have been dissolved.

FIG. 5 is an electron micrograph similar to FIG. 4, with the same type etch, after a five-minute precipitation treatment at 1300 F. but at 10,'000 magnification, which shows several grains and the different orientation of the slip lines as evidenced by carbide precipitates in these grams.

FIG. 6 is an electron micrograph at 10,000x which has been given a carbide precipitating treatment for two minutes at 1300 F. following cold rolling. Comparing this micrograph with FIG. 5, it can be seen that practically the same amount of precipitation of fine carbides and grain boundary carbides has occurred after two minutes as after five minutes, indicating that two minutes treatment at 13 00 F. is sufficient.

FIG. 7 is an electron micrograph at 25,000X of material which has been treated down through the roll flattening step in the manner as has been previously described, and then given a carbide precipitation treatment at 1300 F. for 30 seconds. This material was etched in the same etchant as was used for FIGS. 3, 4 and 5. The attack of the small carbides which have been precipitated at the slip planes is shown; although the amount of carbides is not as great as are precipitated in [five minutes or two minutes as shown in FIGS. 4 and 5, this micrograph does show that carbides are precipitated at this temperature at times as short as one-half minute.

In order to compare the abrasion resistance of material treated according to this invention with other materials, a standard abrasion-resisting measuring test was performed. This test utilizes an annular cast iron surface and a fixture for supporting samples of the material to be tested. Each sample is clamped in the fixture which biases the sample into contact with the cast iron surface, and the fixture is then rotated to cause the sample to slide against the cast iron surface. Kerosene is used as a lubricant, and the surface is kept moist with the kerosene. Various samples are rotated for a predetermined amount of time, and each of the samples tested is compared for the amount of wear caused by this predetermined time of rotation. A control sample of cast iron is used and the wear thereon is arbitrarily assigned a value of 100. The wear on all other samples is compared with the wear on a cast iron sample run against the cast iron surface, and the amount of wear in comparison to the cast iron surface, and the amount of wear in comparison to the cast iron sample is calculated. The lower the number, the better the wear, and hence the better the abrasion resistance of the material. This test was used to measure the abrasion of several samples, and the results are listed in Table I below:

TABLE I Abrasion Material: resistance Cast Iron 100 The material of this invention as cold worked to final gauge without a carbide precipitating treatment 1 5 6 AISI Type 1070 carbon steel 55 AISI Type 440 carbon stainless steel 48 Material of this invention cold rolled 69% in two steps, the first step constituting a 22% reduction and a carbide precipitating treatment between 1000 and 1300 F. 45 Chrome plated carbon steel 44 Carbon 46% manganese 1.18%, silicon .48% chromium 19.18%, nickel 9.35% and the balance iron with incidental impurities The chrome plated carbon steel was selected for comparison in the above test, inasmuch as this material is one of the best wearing, i.e. abrasion resisting, materials for applications where the material requires cold forming and corrosion resistance. The carbon steel is first formed to the desired shape and then plated with chrome. This prior art material has been widely used in many applications.

=It will be readily apparent that the material of this invention for several reasons is superior to chrome plated carbon steel. First, when the chrome plating wears off the low carbon steel, both the abrasion and corrosion resistance are destroyed. Also, the chrome plating must be done after forming, whereas this material is abrasion resistant when it is formed, requiring no further treatment. Further, since the properties are developed prior to forming into the article, there is no tendency to distortion or excessive surface damage caused by treatments after the article has been formed. It should be noted that the carbon content of the alloy as described above must be greater than the solubility of carbon in austenite at 2100 F. Temperatures above about 2100 F. are not normally used in the processing of austenitic stainless steels of this type, and hence there will be excess carbon to form carbides at the highest temperatures used during processing. This will insure that at all processing temperatures there is excess carbon, so that the massive carbides which are required will be present. It should also be noted that various carbide formers such as vanadium, tungsten and molybdenum may be added to improve the type of carbides formed.

Although several embodiments of this invention have been shown and described, various adaptations and modivfications may be made without departing from the scope of the appended claims.

I claim:

1. A process of producing abrasion resistant stainless steel comprising the steps of, providing an austenitic stainless steel having a carbon content in excess of the maximum solubility of carbon in the austenite at 2100 F., alternately cold working and heat treating the stainless steel to produce the desired finished size, said cold working including at least one reduction sutficient to create slip planes in the matrix, said heat treatments including at least one heat treatment within the carbide precipitating temperature range following said one reduction to precipitate carbides on said created slip planes, whereby the carbon in excess of the solubility in austenite will be present as massive randomly distributed carbides, smaller dis continuous carbides precipitated at the grain boundaries and fine carbides precipitated principally on the slip planes.

2. The process of claim 1 wherein the said one heat treatment is performed following the final cold working step.

3. The process of claim 1 wherein the said one heat treatment is performed prior to the final cold working step.

4. The process of claim 3 wherein the said one heat treatment is at sutficient temperature for sufficie'nt time to recrystallize said matrix.

5. A process of producing abrasion resistant stainless steel comprising the steps of, providing an austenitic stainless steel having a carbon content in excess of the maximum solubility of carbon in the austenite at 2100 F., alternately cold working and heat treating the stainless steel to produce the desired finished size, said cold working including at least one reduction of at least 10% to create slip planes in the matrix, said heat treatments including at least one heat treatment within the carbide precipitating temperature range following said one reduction of at least 10% to precipitate carbides on said created slip planes, whereby the carbon in excess of the solubility in austenite Will be present as massive randomly distributed carbides, smaller discontinuous carbides precipitated at the grain boundaries, and fine carbides precipitated principally on the slip planes.

6. The process of claim wherein the carbide precipitating treatment is between 1300 F. and 1700 F. prior to the final cold working step for a time suflicient to recrystallize the matrix.

7. The process of claim 5 wherein the carbide precipitating treatment is between 1000 F. and 1300 F. following the final cold working step.

8. The process of claim 7 wherein the precipitating treatment is for not more than five minutes whereby the matrix will not be recrystallized.

9. The process of claim 7 wherein the precipitating treatment is for not more than two minutes.

10. An abrasion resisting austenitic stainless steel having a carbon content in excess of the maximum carbon solubility in the austenite at 2100 F., said steel having a predominantly austenitic matrix, said matrix having randomly distributed massive carbides, smaller discontinuous carbides precipitated at grain boundaries, and fine carbides within the matrix, said fine carbides being primarily precipitated along slip planes.

11. The stainless steel of claim 10 wherein the matrix has recrystallized after the smaller and fine carbides precipitated.

12. The stainless steel of claim 10 wherein the matrix is unrecrystallized subsequent to precipitation of the smaller and fine carbides.

13. An abrasion resisting austenitic stainless steel having a carbon content in excess of the maximum carbon solubility in the austenite at 2100" F., and having up to about 16% manganese, up to about 2% silicon, from about 10% to 25% chromium, up to about 20% nickel up to about 0.40% nitrogen in a ferrous base, said steel having, a predominantly austenitic matrix, said matrix having randomly distributed massive carbides, smaller discontinuous carbides precipitated at grain boundaries, and fine carbides within the matrix, said fine carbides being primarily precipitated along slip planes.

14. The stainless steel of claim 13 wherein the matrix has recrystallized after the smaller and fine carbides precipitated.

15. The stainless steel of claim 13 wherein the matrix is unrecrystallized subsequent to precipitation of the smaller and fine carbides.

References Cited UNITED STATES PATENTS 2,888,373 5/1959 Cherrie et a1 148l36 FOREIGN PATENTS 601,585 5/ 1948 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner.

WAYLAND W. STALLARD, Assistant Examiner.

U.S. Cl. X.R. 

